The invention relates to a controller for controlling an air-fuel ratio of an internal combustion engine, and more particularly, to a controller for controlling an air-fuel ratio of an internal combustion engine to optimally reduce oxygen excessively absorbed by a catalyst converter.
A catalyst converter for purifying exhaust gas is provided in an exhaust system of an internal combustion engine of a vehicle. When the air-fuel ratio of air-fuel mixture introduced to the engine is lean, the catalyst converter oxidizes HC and CO by excessive oxygen included in the exhaust gas. When the air-fuel ratio is rich, the catalyst converter reduces Nox by HC and CO. When the air-fuel ratio is in the stoichiometric air-fuel ratio region, HC, CO and Nox are simultaneously and effectively purified.
On the other hand, a method for stopping fuel supply when a vehicle is decelerating (for example, when engine braking is used) is known. Such stopping of fuel supply is generally called a xe2x80x9cfuel cutxe2x80x9d. The fuel cut allows fuel efficiency to be improved. The fuel cut is performed, for example, when a throttle valve is totally closed over a predetermined period or longer and the engine rotational speed is greater than a predetermined rotational speed. If the engine rotational speed is below the predetermined rotational speed, or if the throttle valve is opened, fuel supply is resumed.
Since fuel is not supplied during the fuel cut, a large amount of oxygen is introduced and absorbed by the catalyst converter. If the catalyst converter absorbs excessive oxygen, the performance of the catalyst, especially the capability of reducing Nox deteriorates. In order to remove the oxygen absorbed by the catalyst converter, a method for making the air-fuel mixture rich when the fuel supply is resumed is proposed.
Japanese Patent Application Unexamined Publication No. 9-72235 describes a method for feedforward controlling the air-fuel ratio after a fuel cut or lean state is returned to a normal fuel supply state. More specifically, the mass of substances absorbed by the catalyst converter is estimated during the fuel cut or lean state based on output of an air-fuel ratio sensor provided upstream of the catalyst converter. When the fuel cut or lean state is cancelled, the air-fuel ratio is feedforward-controlled to reduce the estimated mass of the absorbed substances.
Japanese Patent Publication No.2913282 discloses a method for determining a target air-fuel ratio for making the fuel mixture rich and a period during which the target air-fuel ratio is maintained. The determination is performed based on the duration of the lean state or fuel cut, and an engine load and engine rotational speed during the lean state or fuel cut. After the lean state or fuel cut is cancelled, the air-fuel ratio is controlled so that the target air-fuel ratio is maintained for the determined period.
Furthermore, a scheme for providing an O2 sensor (exhaust gas sensor) downstream of the catalyst converter is known. When the fuel cut is cancelled, the target air-fuel ratio is set to be rich. A reduction process for the catalyst is started. When the output of the O2 sensor is inverted from a value indicative of lean to a value indicative of rich, the reduction process for the catalyst is stopped.
The mass of substances absorbed by the catalyst varies depending on operating conditions of the engine. If a load of the engine varies, the mass of the absorbed substances also varies. Therefore, it is difficult to precisely determine the mass of the absorbed substances during the fuel cut or lean state.
If the catalyst deteriorates with time, the capability of absorbing oxygen is degraded. After the fuel cut or lean state is cancelled, if the air-fuel mixture is made rich under such degradation, the air-fuel mixture may be made excessively rich. Such an excessive rich state increases HC and CO emissions.
Thus, the feedforward control of the air-fuel ratio is unstable against variations in operating conditions of the engine and variations in degradation of the catalyst. The feedforward control may degrade the purification performance of the catalyst.
There exists dead time in combustion cycles of the engine and transportation through the exhaust system. It takes some time from adjustment of an amount of fuel injection based on a target air-fuel ratio determined from the output of an O2 sensor until the result of the fuel injection is reflected in the output of the O2 sensor. Therefore, if a process for making the air-fuel ratio rich is stopped in synchronization with the inversion of the O2 sensor provided downstream of the catalyst from lean to rich, the catalyst may be excessively reduced. As a result, the amount of HC and CO emissions is increased.
Therefore, there is a need for air-fuel ratio control that performs a reduction process that is stable against variations in a load of the engine after a lean state or fuel cut is cancelled. Furthermore, there is another need for air-fuel ratio control that performs a reduction process in accordance with deterioration of the catalyst. There is yet another need for air-fuel ratio control that prevents the air-fuel ratio from being made excessively rich after a lean state or fuel cut is cancelled.
According to one aspect of the invention, an exhaust gas sensor is provided between an upstream catalyst disposed upstream of an exhaust manifold and a downstream catalyst disposed downstream of the exhaust manifold. A virtual exhaust gas sensor is virtually provided downstream of the downstream catalyst. When a lean state is cancelled or when a fuel cut is cancelled, the controller estimates an output of the virtual exhaust gas sensor based on a gas amount that contributes to reduction of the upstream and downstream catalysts, and an output of the exhaust gas sensor. First air-fuel ratio control controls the air-fuel ratio of the internal combustion engine in accordance with the estimated output.
According to the invention, it is possible to control a purification atmosphere (oxidation atmosphere and reduction atmosphere) of the downstream catalyst that can not be directly observed by the exhaust gas sensor provided between the upstream and downstream catalysts. The reduction process for the downstream catalyst is appropriately and stably performed based on the estimated output of the virtual exhaust gas sensor. Thus, the purification rate of Nox after the lean state or fuel cut is cancelled can be quickly returned to an optimal rate.
According to another aspect of the invention, the gas amount that contributes to reduction of the upstream and downstream catalysts is determined based on an operating condition of the engine. Therefore, a variation in the load of the engine after a lean state or fuel cut is cancelled, a variation in the duration of a lean state or fuel cut, a variation in the air-fuel ratio during a lean state or fuel cut, and a variation in deterioration of the catalysts are compensated. As a result, the purification rate of Nox after a lean state or fuel cut is cancelled can be stably returned. Furthermore, the air-fuel ratio is prevented from being made excessively rich caused by an excessive reduction process, avoiding increasing the amount of HC and CO emissions.
According to another aspect of the invention, when a lean state is cancelled or when a fuel cut is cancelled, the controller changes a target air-fuel ratio to a predetermined rich value. The gas amount that contributes to the reduction of the upstream and downstream catalysts is determined based on the amount of the change in the target air-fuel ratio. According to one embodiment, the target air-fuel ratio is controlled to change from a stoichiometric state (stoichiometric air-fuel ratio) to a predetermined rich state. In this case, the gas amount that contributes to the reduction of the upstream and downstream catalysts is determined based on a difference between the target air-fuel ratio and the stoichiometric air-fuel ratio. Since the air-fuel ratio that contributes to the reduction of the catalysts is taken into consideration, the accuracy of the estimation of the output of the virtual exhaust gas sensor is improved.
According to another aspect of the invention, the estimated output of the virtual exhaust gas sensor is expressed by a binary digit indicating lean or rich with respect to a predetermined value. Thus, a computational load for estimating the output of the virtual exhaust gas sensor is reduced. The predetermined value is, for example, the stoichiometric air-fuel ratio.
According to another aspect of the invention, the estimated output of the virtual exhaust gas sensor is a future value. The future value temporally precedes a value that would be detected by the virtual exhaust gas sensor if the virtual exhaust gas sensor were actually mounted downstream of the downstream catalyst. Since the air-fuel ratio is controlled in accordance with the future value, an excessive reduction process caused by dead time included in combustion cycles and transportation through the exhaust manifold is prevented.
According to another aspect of the invention, the controller further performs second air-fuel ratio control for controlling the air-fuel ratio based on the output of the exhaust gas sensor provided between the upstream and downstream catalysts. The second air-fuel ratio control allows functions of the upstream and downstream catalysts to be effectively and selectively used, implementing the optimal purification rate of the catalysts. The first air-fuel ratio control and second air-fuel ratio control are switched in accordance with a predetermined condition. The predetermined condition includes a condition in which the estimated output of the virtual exhaust gas sensor is inverted from lean to rich. When the reduction process for the catalysts in a state in which the air-fuel ratio is enriched is ended, the first air-fuel ratio control is completed, and the second air-fuel ratio control is started.
The second air-fuel ratio control allows deleterious substances to be effectively removed by the upstream and the downstream catalysts. The first air-fuel ratio control allows a large amount of oxygen absorbed by the catalyst converter during a lean state or fuel cut to be effectively reduced. Therefore, the catalysts are prevented from deteriorating due to an oxidation atmosphere while the purification rate of the catalysts are optimally maintained.
According to another aspect of the invention, the second air-fuel ratio control has an integration term in a manipulated quantity for manipulating the air-fuel ratio. The calculation of the integration term is prohibited when the air-fuel ratio is controlled by the first air-fuel ratio control. That is, while the reduction process for the catalysts is performed, the integration term is held. Thus, when the second air-fuel ratio control is resumed, the air-fuel ratio control is prevented from becoming unstable due to an excessively increased integration term.
According to another aspect of the invention, the second air-fuel ratio control identifies a parameter used to determine the air-fuel ratio in each cycle. The identification of the parameter is prohibited when the first air-fuel ratio control is performed. When the second air-fuel ratio control is resumed, the air-fuel ratio control is prevented from becoming unstable due to an improper parameter.
According to another aspect of the invention, the second air-fuel ratio control further limits a manipulated quantity for manipulating the air-fuel ratio within a predetermined range. The second air-fuel ratio control variably updates the predetermined range in accordance with the manipulated quantity. The update of the predetermined range is prohibited when the first air-fuel ratio control is performed. Thus, when the second air-fuel ratio control is resumed, it is avoided that the exhaust gas sensor can not be controlled toward a predetermined target value because of the manipulated quantity limited by an improper predetermined range. It is avoided that the purification rate of the catalysts is extremely impaired.
According to another aspect of the invention, the controller accumulates gas amounts that contribute to the reduction of the upstream and downstream catalysts in respective cycles. A gas amount for reducing the upstream catalyst is determined in response to the inversion of the output of the exhaust gas sensor provided between the upstream and downstream catalysts. A total gas amount necessary to reduce both the upstream and downstream catalysts is determined based on the determined gas amount for reducing the upstream catalyst. The output of the virtual exhaust gas sensor is manipulated to indicate a completion of the first air-fuel ratio control if the accumulated gas amounts reach the determined total gas amount. Thus, both the upstream and downstream catalysts are appropriately reduced. The purification rate of Nox after a lean state or fuel cut is cancelled can be quickly and stably returned.